As Sean said: The power amp is the final stage of amplification which drives the output transducers -- the speakers in an audio system, servos in a robot, whatever.In an audio system there are usually other stages of amplifiers -- preamplifiers, or preamps -- which come before the power amp to bring the signal up to a comfortable level for processing by things like your stereo's tone controls or a sound system's mixer and effects. The output of that processing goes to the power amp.
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A power amp usually requires a separate Preamplifier to condition input signals prior to being amplified by the power amp and sent to the speakers. For instance, the audio signal from a guitar is different than the signal from a CD player, different from that sent by a phonograph, and different from that sent by a tape deck. Each requires specific signal conditioning prior to being sent to the power amplifier which drives the speakers.A preamp by itself will not power the speakers. A power amp without a preamp usually will do a very poor job of amplifiying the signal from most sources.An audio amplifier (as in a stereo amp or similar) incorporates both a preamplifier and a power amp. in one package.
So the answer to your second question is YES, but you'll need a preamp between the audio source and your power amp. I've successfully used a stereo amp (ie, a consumer grade stereo system) as a preamp in the past for work with soundcard generated midi music that was then fed to my PA, ie, Power Amp.
Aw shucks, I didn't even really answer the first question. A power amp drives the speakers. That's it's whole job. It is sized for the expected speaker load.All the filters and boosting of signal levels to one usable by the power amplifier is accomplished by the pre-amplifier.
A power amplifier is a very loud amp for a speaker (LM1875, LM675) which they use for concerts and disco parties. You need a fairly good preamp and sometimes if it is a really loud power amp then you need a amp in between to detect the small signals from the preamp. The microphone you see in speeches has a preamp.
thank you all
Also note the smaller ELECCIRCUIT can only drive an 8 ohm speaker,While the AN7117 is driving the more powerful 4 ohm speaker.